1. Field of the Invention
The present invention relates to communications systems in general and more particularly to electro/optical communications systems wherein the receiver restores transmitted signals to the condition they were in at transmission.
2. Prior Art
Optical communications systems that transmit digital and analog signals are well known in the prior art. A typical communications system includes a transmitter with an optical source such as, for example, a light-emitting diode for converting an electrical input signal to an optical signal. An optical fiber transmits the optical signal to a receiver. An optical photodetector converts the optical signal back to an electrical signal. The electrical signal is amplified by a circuit arrangement and is given to a utilization device. The systems are attractive because their radiation is in acceptable range even when information (data) with large bandwidth are transmitted.
One of the problems that is associated with optical communications systems is that the transmitted signal is subjected to much more distortion than a similar transmitted via a conventional copper wire communications channel. It is believed that the distortion is caused by the characteristics of the optical components. The components usually include transmitting diodes, light receiving photodetectors, optical connectors, optical fibers, etc. As these components age, their characteristics change and contribute to the non-linearity problem.
Another problem that is common in optical channels is that the gain is non-uniform. Non-uniform channel gain is particularly harmful to video channels which transmit information that is subsequently displayed on a monitor or cathode ray tube (CRT). In the case of monochromatic systems, only one channel is used to transmit the video data. Usually, for each transmission burst or interval a line of data is transmitted. The quality of the displayed data is a function of the channel gain. Therefore, for good quality display it is desirable to maintain a constant channel gain.
The problem is even more severe with color transmission. Unlike monochromatic systems that require a single optical channel, color systems require three channels to transmit the red, green and blue colors required for color display. Therefore, with color transmission it is desirable that the gain be maintained at a uniform level across the channels.
The prior art has used two approaches to address the signal distortion and non-uniform gain problems. In one approach the prior art seeks to improve the operating characteristics of individual components used in the optical channel. It is believed that if these characteristics are improved, the proclivity of the optical channel to distort the optical signal will also be reduced. More particularly, the light-emitting diodes and the light-receiving photodetectors are perceived as potential problem components and circuits are provided to compensate accordingly.
One noted problem with these diodes and/or photodetectors is that these devices change their operating characteristics as the junction temperature changes. Usually, the junction temperature is a function of the modulating signal. Stated another way, the junction temperature differs according to whether the modulating signal is at low or high frequency. U.S. Pat. No. 4,443,890 describes a direct-light modulation information transmission system wherein filtering means are provided at the emission side and the reception side for compensating for non-linear thermal effects of the emission means and the linear thermal effects of the reception means. In the patent the emission means is a light-emitting diode (LED) and the reception means is a photodiode.
U.S. Pat. No. 4,654,891 is another prior art patent which addresses variations, due to temperature, in the characteristics of the optical emitter and receiver. In particular, the subject patent describes a video transmission system in which a pre-distortion circuitry is provided at the transmitter. The circuit dynamically modifies the signal level with respect to which the video signal is clamped. An automatic gain control (AGC) circuitry, at the receiver, detects a DC level in the received video signal. A shift in the DC reference level of the received video signal causes the AGC circuitry to correct the receiver gain.
Even though the above prior art devices work well for their intended purposes, it should be noted that the problems which are associated with the optical channel are caused by the different components which are used in the channel. Thus, by correcting problems associated with only one of the problem sources would not necessarily correct the problem associated with the entire optical channel. To correct the problem which affects the entire optical channel requires a more comprehensive approach.
U.S. Pat. No. 4,742,575 to Arita et al is an example of prior art patents in which a comprehensive approach is used to correct problems in the optical channel. The patent describes a light signal transmission/reception system with a means which superimposes predetermined different levels reference pulse signals on the informational signals. The receiver is provided with a means for deriving the amount of change in the reference pulse signals. The derived change is used to compensate the magnitude of the received signal. The reference pulse signals are then filtered from the informational signals.
U.S. Pat. No. 4,249,264, Crochet et al, is another patent in which a low frequency reference signal is superimposed on an informational signal at the transmitter. At the receiver, the received signal is passed through a high pass filter which delivers the useful signal to a utilization device and to a low-pass filter which extracts the reference signal. The reference signal is compared with a predetermined voltage and an error signal is generated which is used for polarizing or biasing the reception diode.
There are certain applications in which the Arita et al and Crochet et al patents are not suitable for use. In particular, these references raise several additional problems if used in video transmission systems. As video transmission systems exist today, there are no practical low frequency reference signals which can be superimposed without disturbing the video signal which is displayed. With regard to the Arita et al patent, another problem is that the range (high and low) of the reference signals must be sufficiently different from the information signal so that the reference signals can be detected at the receiver. This suggests an unnecessary wide bandwidth whose upper limit can only be used to transmit reference signals as opposed to informational signals.